Capacitive biofeedback sensor with resilient polyurethane dielectric for rehabilitation

ABSTRACT

A capacitive biofeedback sensor including a polyurethane dielectric sandwiched between two wire mesh or carbon impregnated silicone rubber conductors. The sensor is placed within a patient&#39;s shoe, boot, ankle, brace, crutch, hand grip, wheelchair, etc., and provides biofeedback to help patients relearn function or prevent complications that impede function.

This application is a division of U.S. patent application Ser. No.07/908,121 filed Jul. 01, 1992, now U.S. Pat. No. 5,449,002.

BACKGROUND OF INVENTION

This invention relates to apparatus for medical and therapeuticapplication and, more particularly, to a sensor for providingbiofeedback signals to help patients relearn various physical functions,or to prevent complications that impede these physical functions.

Frequently, it is desirable to observe the mechanical pressure appliedin performing some otherwise routine physical function, not only by apatient suffering from some illness or disability, but also from arecovering patient, who is trying to regain, or at least to improve somephysical function. This need to observe the mechanical pressure appliedby the patient in performing a particular function also extends topatients who require braces or other orthotic apparatus, as well as toprosthetic equipment, or artificial limbs, and assist devices of whichcrutches, wheelchairs and the like are typical.

Unconscious or involuntary bodily processes, for example, the pressuredistribution applied by the sole of a foot to the inner sole of a shoewhile walking, the action of the ankle, also while walking or, theapplication of bodily weight to a pair of crutches and the like, if madeperceptible to the senses can in many instances be modified throughconscious mental control to produce a proper gait, or to develop thecorrect use of crutches in order to avoid impairing those nerves thatcontrol arm movement and sensation, the brachial plexus. This treatmenttechnique, often referred to as biofeedback, is beneficial.Unfortunately, known biofeedback techniques and equipment areunsatisfactory for a number of reasons and, as a consequence are not ingeneral use by patients either in acute care, rehabilitation or at homein spite of the need for devices of this nature. However, the need forthem is compelling.

For instance, biofeedback may provide for monitoring the weight appliedto a limb ("limb load monitoring"). An important example of this limbload monitoring relates to limiting the weight that is borne by the limbwhen recovering from leg fracture or joint replacement. Total hipreplacements alone are performed 200,000 times per year in the UnitedStates and "toe-touch" or "partial" weight bearing is almost alwaysprescribed for the recovering patient by the surgeon. Partial weightbearing continues until healing has progressed sufficiently to allow thepatient to apply full weight to the recovering member safely. In almostall rehabilitation centers, however, there is no way to assurecompliance with this prescribed treatment except through observation bytrained personnel. Although the true cost of excessive weight bearing inthese circumstances does not seem to have been systematically studied,it is nevertheless reasonable to assume that this failure tosystematically control the weight borne by a recovering limb prolongshealing time, increases the duration of hospitalization and patientdependent status. Thus, it seems that a reliable apparatus forsystematically monitoring the load applied to a recovering limb couldsignificantly reduce this apparent loss in patient recovery time and theeffort required of trained therapists as well as reducing inefficientuse of medical facilities and attendant expense.

In addition to the need for improved rehabilitation techniques,described above, there are a number of applications of limb loadmonitoring to gait training. Limb load devices would help amputees toapply weight to prosthetic limbs in a symmetrical manner as a prelude toprogressive ambulation. Children with cerebral palsy might be taught towalk correctly by placing weight on the heel and stop walking with anequine, or downwardly pointing ankle. In this way, inpatientrehabilitation service time could be reduced, and safe ambulationpromoted.

Other potential applications include rewarding the patient for meetingother strength goals. Illustratively, developing muscular contractionswithout significantly shortening the muscle fiber in the hand or knee,that is, isometric grip strength and knee strength might be improvedthrough biofeedback technique. Further, individuals with rheumaticdiseases also could greatly benefit from effective limb load monitoring.

Another group of patients, suffering from disabilities that confine themeither permanently or temporarily to a wheelchair also could benefitfrom a practical application of biofeedback to their status.Paraplegics, persons with spina bifida and others who lack feeling orsensation below the waist would benefit from a biofeedback signal, cuingthem to shift their weight on the seat of the wheelchair and reduce thepossibility of sacral pressure ulcers. Still another group of patients,bound to wheelchairs, are those suffering from mental deterioration andstrokes. In either instance, falling from a wheelchair is anall-too-common injury for patients of this nature and some biofeedbackmechanism that could lock the wheelchair brakes before rising from thewheelchair would reduce injury and promote safety awareness.

Naturally, a great deal of effort has been applied to develop apparatusthat will provide a reliable, inexpensive and accurate biofeedbackdevice. These efforts involved a number of technologies that can becategorized, generally, as capacitive, resistive, hydraulic, pneumaticand also of a general, miscellaneous character for providing an accuratemeasure of the pressure applied by a patient to a surface.

Capacitive transducers for biofeedback operation have inherent benefitsin monitoring the loading on limbs or on other body parts. As the weightof a patient is applied to a measuring capacitor, there is a relativelylinear increase of capacitance with applied weight, which permits theuse of simple, low cost electrical circuits to generate a signal that isdirectly proportional to the applied weight. In this way, a capacitivetransducer might be developed to measure the total weight that isapplied to complex, irregular surfaces. In spite of this potential,capacitive transducers have been overlooked during most of the pastdecade in favor of resistive devices. What has been lacking, and madecapacitive transducer research for this purpose unattractive has beenthe lack of an acceptable dielectric.

The dielectric is the insulating middle layer that is interposed betweenthe electrically conductive plates that comprise the capacitor.Ordinarily, the dielectric properties remain constant as a force, suchas that applied by a limb, compresses the dielectric and brings theconductive plates closer together thereby causing the capacitance of thesensor to increase measurably.

Many dielectrics, however, are subject to creep, which is the long termchange in the dielectric thickness after initial loading. Dielectricsalso are subject to hysteresis, which is the difference in compression,or capacitance, at a given load with the load increasing, and with theload decreasing. The sensitivity of the dielectric, or the ratio ofcapacitance in the loaded and unloaded state, per application of unitweight also is an important parameter. The speed with which the sensorachieves steady state after weight is applied, or the dynamic responseof the dielectric and the resiliency of the dielectric, which is theability of the dielectric to retain its unloaded thickness and baselinecapacitance after a long duration of static or dynamic loading also aresignificant factors in choosing suitable dielectrics. All of theseproperties are a function of the dielectric material composition andstructure. Ideally, for the purpose of a biofeedback sensor, adielectric should enjoy zero hysteresis and creep with a very highresilience, sensitivity, and dynamic response.

The Krusen Limb Load Monitor is an illustrative prior art device. Thisdevice is described in Craik, R. and Wannstedt, G., Proceedings 2ndConf. Devices and Systems for the Disabled, 1975, 19-24. The capacitivetransducer consists of three layers of copper- Mylar® laminate the outertwo layers of which are separated from the inner layer each by a layerof "closed cell foam tape". The whole device, as shown in the paper, hasthe shape of an inner-sole. The characteristics of the dielectric,however, were not published. This-device nevertheless fails to provide afully satisfactory capacitive transducer because it lacks adaptabilityto orthotics (i.e., braces), assistive devices or wheelchairs and it isexpensive.

Additionally, if conventional polyethylene foam tape is the dielectricthat is used in this apparatus, the capacitive transducer also should besubject to significant hysteresis and creep, and would be unsatisfactoryafter repeated compression which would require frequent replacement.

An apparent improvement in the foregoing device, in which two layers ofNeoprene sponge, laminated between three layers of copper foil isdescribed in Miyazake, S., and Ishida A., Medical and BiologicalEngineering and Computing, 1984, pp. 309-316. Unfortunately, Neoprene,like polyethylene, has limited resilience to static and dynamic loadingand also is unsatisfactory with repeated use.

On initial testing there was good agreement between force plate andtransducer data on patients with foot shapes that were not markedlyabnormal. However, higher local pressures from deformed feet causedexcessive error (i.e., overestimation of weight). To reduce this errorstiff plastic was built into the laminate to spread out area of forceapplication. This modification, however, produced a device that wascumbersome, thick and stiff and which prevented the capacitivetransducer from acceptable adaptation to shoes, orthotics, assistdevices or wheelchairs.

A second major category of sensors for rehabilitation are resistivedevices, which appear to be the most popular approach to solving thebiofeedback sensor problem. In these resistive devices a change inelectrical resistance is related to the force applied by the patient toeffect that change.

Force Sensitive Resistors (FSR's) are marketed by Interlink Corp. Theyare thin (0.25 mm thick), physically flexible, inexpensive devicesavailable in 1 cm² squares. Externally there are two adjacent Mylar®sheets. On one Mylar sheet is a high resistance polymer or carbon film,and on the other Mylar sheet an interdigitated or interlocked pattern ofopen ended conductors is formed. Ruggedness appears to be good withextended use for this transducer and it may be used with simpleelectrical circuits.

For these reasons FSR's have been proposed for "everyday" rehabilitationpressure measurement in several applications. These include monitoringpressure under insensate feet in patients afflicted with diabetes andleprosy (M. Maalej, et. al., IEEE, 9th Conference of the Engineering inMedicine and Biology Society, 1987, pp. 1824, 1824). A finger and handexercise device and "spot" high pressure sensor to prevent sacralpressure ulcers have also been described (Hyman, W. et. al., RESNA, 13thannual Conference, 1990, pp. 201, 202). It additionally has beenproposed for motorized prosthesis control (Heckathorne, C., RESNA, 12thannual Conference, 1989, pp. 224-225).

These resistive devices, however, are unsatisfactory for severalreasons. Illustratively, the response of these resistive transducers isnot linear but is substantially exponential. Unloaded resistance iseffectively infinite, and resistance declines rapidly as weight in therange from 10 to 20 pounds is applied. Above this range, the electricalresistance approaches an asymptotic minimum value. Finally, as thisapplied weight exceeds 50 pounds the change in electrical resistance maybe obscured by electrical background noise. In terms of noise, moreover,the device demonstrates significant hysteresis that varies from deviceto device and from trial to trial with the same device.

A microprocessor and, less satisfactory, an integrated circuit can beused to produce a linear response in which the electrical output isproportional to the applied pressure. To determine the weight applied toa large irregular contour, the output signals from many linearizedsensors must be summed, and this use of more sensors makes accuracy lessdependent on variations of contour. This solution to the problemincreases the complexity of the associated electrical circuits, reducesreliability and dramatically increases current drain which decreases thebattery life for portable devices. Because of battery drain alone, FSR'swould be unacceptable for use in a wheelchair embodiment.

To use a minimum number of sensors, FSR's must be placed at points ofmaximum pressure. For instance, for limb load monitoring, sensors may beplaced on the first and fifth metatarsals and the heel. Should a patienthave an irregular foot contour or wear a cast, the resistive sensorplacement must be customized.

Biofeedback for hand grip strength application using FSR's presents aneven more serious problem, as hand size and placement are difficult tocontrol. Thus, FSR's lack flexibility where total pressure is to beobtained over irregular or complex surfaces. In contrast, capacitivedevices are not so inflexible or cause as much current drain. Indeed,FSR's and other resistive devices are best used, not in a dynamicenvironment between the patient and the therapeutic or prostheticdevice, but in static situations in which local pressures are to bemapped and quantified over two dimensions, e.g. to identify skin areasthat are at risk for breakdown and ulceration. Calculated forecastsbased on resistive transducer data, may, for example, guide theprescription of custom shoes.

One such resistance device is based on the flexible force sensordeveloped by Polchaninoff in 1984 and described in U.S. Pat. No.4,426,884. The electrical behavior of this sensor is similar to that ofthe FSR described above. Twenty of these sensors, moreover, areincorporated into the "multi-event notification system for monitoringcritical pressure points on persons with diminished sensation of thefeet" shown by Goforth in his 1987 U.S. Pat. No. 4,647,918. In theGoforth disclosure, resistors are placed under the heel, lateral plantarsurface, metatarsal heads, and toes of the patient. The informationsignals from each of these resistors is transmitted to a portablemicroprocessor. The complexity, cost, and fragility of this apparatusmake it unsatisfactory, however, for application to limb loading afterorthopedic procedure or for gait training.

Tecscan Inc. (Boston, Mass.) has developed a pressure mapping system forgait analysis (Sensors, May 1991, pp. 21-25). in which 960 resistiveelements are continuously sampled by a personal computer duringambulation. Graphics of excessive local pressure on the foot aredisplayed in real time. A similar device also has been developed byTecscan for determining gluteal, or buttocks pressure contours. Thislatter system may be used for evaluating seating systems. Both of thesesystems, from the standpoint of a suitable biofeedback apparatus in adynamic environment are expensive, lack durability, are fragile andcomplex.

Hydraulic devices easily determine total weight applied to irregularsurfaces and, in this respect, are similar to capacitive devices.However, by definition these hydraulic devices measure the total weightby means of fluid pressure in a closed chamber. As a consequence,hydraulic devices tend to be large, heavy, bulky, thick or rigid.Nevertheless, hydraulic and conceptually similar pneumatic systems havebeen applied to limb load detection, grip strength detection, andseating pressure systems.

Two devices for providing objective and immediate limb loading datafeedback are described in Sipe, U.S. Pat. No. 3,974,491 and Pfeiffer,U.S. Pat. No. 3,791,375. Both of these devices employ reservoirs belowthe heel and sole. Applied weight compresses these reservoirs and theforce, or weight is monitored by a transducer on the patient's ankle.Apart from the need for a dedicated shoe, these transducers aredifficult to put on and take off, and the height added to the shoe toaccommodate these devices may impede ambulation in an individual whosewalking is already impaired and possibly unsafe.

Hydraulic devices for measurement of grip strength are known asdynamometers. These devices are large and heavy and the applied force isread directly from a pressure gauge, which is difficult for the patientto read. Due to these limitations, they are not designed for use duringexercise and are certainly inappropriate for the elderly or frailarthritic who would most benefit from grip strengthening.

Pneumatic devices also have been developed to measure pressures beneathbony prominences in evaluating wheelchair seats. Because these pneumatictransducers are essentially flattened, modified balloons, their size,shape and fragility make them inappropriate for permanently monitoringthe weight of a patient on a wheelchair seat and they are not adaptableto the wide variety of wheelchair and wheelchair cushions now available.

Other physical phenomena have been employed, apart from the fourmentioned above, to provide a measure of the physical pressure appliedby a patient to some therapeutic or prosthetic device. Exemplary of thislast group, the mechanical switching device, described in Gradisar, U.S.Pat. No. 3,702,999 measures limb loading. Each of two switches has twometal discs separated by an "O" ring. As weight is applied to the topdisk, the "O" ring is compressed until electrical contact is establishedbetween the two discs to energize a buzzer. By adjusting a set screw onone disk, the weight at which the buzzer sounds also may be adjusted.This device is subject to mechanical problems; it is difficult to adjustthe device to the "trigger" weight; and only one type of shoe (a "castboot") may be used with this apparatus.

Other phenomena applied to biomedical force measurement include straingauge and piezoelectric technologies. Strain gauges are made of coils ofthin high-resistance wire or of metal or silicon impressions that arediffused or otherwise applied to a substrate. Because they enjoy a highlevel of accuracy, these transducers are good for gait research.Nevertheless, they also are fragile, expensive and consume large amountsof power. For these reasons, devices of this nature also are unsuitedfor the demands of daily therapeutic use. Additionally, these devicesemit low power signals that require extensive amplification, a largevolume, a controlled environment and are expensive.

In summary, weight bearing biofeedback apparatus and proposals that havecharacterized the prior art are undesirably complex, expensive, subjectto failure, and lack durability. Although force sensitive resistors arepromising for certain limited applications, they are substantiallynon-linear in electrical response and need dedicated electrical circuitsthat generate linear and integrated output signals that reflect themeasured weight. For large, irregular surfaces an array of many FSR'smust be used. This further increases the complexity and the cost for thesystem as well as the current drain while, at the same time decreasingdurability and battery life.

Capacitive transducers, on the other hand are inherently linear andintegrate the force information. Capacitive transducers further areindependent of the body and transducer contours. These devices need notbe customized and thus they are readily interchanged among patients.

On this basis, it seems that capacitive transducers should have the bestpotential for biofeedback operation except for the fact that no suitabledielectric has been identified that will permit the goals of simplicity,low cost, reliability, portability and low current drain to be achievedin a commercial device.

SUMMARY OF INVENTION

The present invention overcomes the disadvantages of the prior art. Itis a capacitive transducer, consisting of an open cell polyurethane foamdielectric sandwiched between two conductors. This foam is a key featureof this invention. It is used in every embodiment. In most embodiments,the conductors are wire mesh, similar in appearance to window screen.

A second un-obvious feature of this invention is use of carbonimpregnated silicone rubber (CISR) as the conductors in someembodiments. CISR is the material employed in transcutaneous electricnerve stimulation (TENS). It has high resistance to DC current. However,for measurement of capacitance it acts as a conductor. Embodiments withCISR have remarkable pliability and ruggedness.

The preferred polyurethane foam is characterized in terms of chemistry,density, firmness and morphology. Chemically, polyurethanes are formedas diisocyanates (having the general structure R--N═C═O) react withpolyols (alcohols). Density is commonly referred to as pounds per cubicfoot, or pcf. The preferred density range for the present invention is5-50 pcf. Firmness, is defined as the compressive force (expressed aspounds per square inch, or psi) required to cause a 25%. compression ofthe polyurethane foam. (A device causing a 0.2 inch/minute strain rateis used). The preferred firmness range for the instant invention is0.1-100 psi. Morphologically, the foam is made up of a plethora ofapproximately spherical cells. The preferred average cell diameter is50-200 microns. Cells are in communication through a multitude of pores(thus, "open cell"). Unimpeded air flow through the foam substance helpsestablish resilience. On formal testing there is a loss of 5% thicknessor less on an ASTM 1667 compression set at 73 degrees F. Foams of thistype are preferably manufactured by Rogers Corp., Poron MaterialsDivision, East Woodstock, Conn.

The preferred polyurethane foam of this class is marketed as PPT®(mnemonic for Personal Protective Technology), a trademark of LangerBiomedical Group. This open cell, high density, microcellularpolyurethane foam has an average pore size of 100 microns, density of 20pcf, and firmness of 10±3 psi at 25% compression. Polyurethane foams ofthe present invention are preferably constructed with PPT® dielectric.Thus, in the discussion below open cell polyurethane foam is referred toas PPT®. However, any member of the same class (with the above describedcomponents) may be used where appropriate. PPT® (unlike other open-cellfoams) has been well-known in the field of rehabilitation for severalyears. The conventional application is as foam padding for protectiveuse in shoes. Although well known in the field of rehabilitation forseveral years, no one has thought to apply it to solve the weightbearing dilemma. However, the same features that make it a goodprotective foam cushion with excellent energy absorptive properties alsomake it the best available capacitive dielectric. These includeunsurpassed resiliency to both static and dynamic loading, and linearitywith compression.

PPT® was found superior to neoprene and polyethylene foams in resiliencyon extensive dynamic testing (Brodsky, et. al., Foot and Ankle, Vol. 9,Dec., 1988, pp. 111-116). PPT® showed no loss of thickness for 10,000cycles of compression or shear compression. (10,000 cycles correspondsto 9 hours of walking at normal cadence), whereas Neoprene showed a5-15% loss of thickness, and polyethylene foam 15 to 50% loss ofthickness. Devices made with PPT® should thus last a very long time.Further, one expects devices made with PPT® be far more durable thanpolyethylene (Krusen Monitor) or neoprene (Miyazake and Ishida device).

Furthermore, testing reveals PPT® to have far better resiliency tostatic loading than polyethylene and neoprene foams. A square of PPT maybe placed under a desk leg for a week and return immediately to itsoriginal shape. However, neoprene or polyethylene foams would remainpermanently compressed.

The second advantage of PPT is improved linearity with compression. Itis fortuitous but gratifying that this transducer is linear withinnarrow limits from zero to 75% compression of the dielectric. The rangeof linearity of pressure to compression is wider than the other two foammaterials. Thus, deformed feet transferring high local areas ofcompression should yield equivalent capacitance change to a more spreadout load. This is a clear improvement over the device described byMiyazake and Ishida.

Additionally, creep is imperceptible with greater than 75% compressionfor extended periods. This is in contrast to neoprene and polyethylenefoams, which show significant creep at this level of compression.

Thus, this laminate is very promising as basis for a variety of weightbiofeedback devices in the clinical setting. Seven specific embodimentsare introduced in the following paragraphs:

A. Partial Weight Bearing Or Cast Boot Embodiment

The purpose of the cast boot embodiment is to improve compliance withtoe touch or partial weight bearing orders after bone surgery on the hipor leg. This assures the shortest, safest course of post-operativerehabilitation. A cast boot is an oversized, open, canvas shoefrequently worn by such patients during daily ambulation training. Athin embodiment may fit inside a cast boot. In the preferred embodimentthe conductive layer is wire screen. However, carbon impregnatedsilicone rubber may be used.

B. Shoe Embodiment

The shoe embodiment is intended for a patient with an insensateextremity. The well known work of Bach-Y-Rita suggests that electricallyaugmenting sensory feedback helps such a patient re-learn and maintainsafe, functional ambulation. Conditions that lead to insensateextremities include peripheral neuropathy (e.g., from diabetes) ortraumatic spinal cord injury. A secondary benefit may be reduction infoot ulceration and deformities. Many prosthetic wearers would benefitfrom sensory feedback, especially the many amputees with diabetes.

Physically, the embodiment resembles an inner-sole and is concealedwithin a shoe. This device is best employed where cosmoses and highlevel functioning are paramount. Use is for an indefinite period due toa chronic condition. Low cost and long life are likely to create amarket for this embodiment. In the preferred embodiment the conductivelayer is wire screen. However, carbon impregnated silicone rubber may beused.

C. Ankle Brace Embodiment

Children with diplegic cerebral palsy tend to ambulate with an equinus(i.e., downward pointing) ankle due to poor cerebral processing of motorinformation. They adopt normal heel strike only when constantly reminded(i.e., cued). This behavior persists despite use of an AFO (ankle footorthosis, or ankle brace), specifically designed to prevent it. Anembodiment is described which allows electronic cuing of heel strike. Itis designed to be used with an AFO. Previous work (Seeger, B. R.,Archives of Physical Medicine and Rehabilitation, 56: 237-240) suggestssuch a device would improve ambulation in these children. In thepreferred embodiment the conductive layer is wire screen. However,carbon impregnated silicone rubber may be used.

D. Grip Strength Embodiment

Isometric strengthening (i.e., exercise of a muscle without change ofits length) is preferred in treatment of arthritis and tendinitis of thehand. This is because joint and tendon movement tends to worseninflammation. The grip strength embodiment measures grip pressurewithout perceptible movement of the device. This is due to thecharacteristics of the polyurethane dielectric in the capacitivetransducer. It is designed to replace the heavy and cumbersomedynamometer that works on hydraulic pressure. The present invention issmall and lightweight, and thus may be used by even a weak, frailpatient without supervision. By giving continuous feedback about forceapplied, it will improve motivation and quality of exercise in thesepatients. It may also be used for strengthening and edema control in theweak hand of stroke patients, the hand that tends to be neglected (i.e.,ignored) due to the cognitive effects of the stroke. Lastly, such adevice could be used by the lay public for exercise of grip.

E. Axillary Crutch Embodiment

Axillary crutches are frequently used by patients that are partialweight bearing on one leg. They transmit weight through an arm and handto the brace instead of through the leg. However, if the crutches areused incorrectly, the brachial plexus may be injured. (The brachialplexus is a bundle of nerves passing through the axilla to control armmovement and sensation). Months of pain, weakness and numbness mayresult from this not uncommon injury. Strain gauges normally retail for$500.00 on up, so this device is not very cost-effective. However, thepresent invention has a simple construction that may enable its routineuse as a safety feature within axillary crutches at relatively low cost.

F. Wheelchair Embodiment

An inexpensive, durable means to detect pressure on a wheelchair seatwould have many uses. Paraplegics and patients with spina bifida,insensate below the waist, would benefit from a device to cue weightshifting at regular intervals. It might reduce incidence of sacralpressure ulcers in these populations. Malament, et. al. (Archives ofPhysical Medicine and Rehabilitation, 56: 161-165) demonstrated suchbehaviors could be learned by use of biofeedback. However, bulkyhydraulic devices or expensive strain gauges have been used in the pastfor monitoring. Other devices are based on switches, which do not allowthe patient to learn progressive weight reduction during the weightshift. The wheelchair embodiment is unobtrusive, relatively inexpensive,readily adaptable to any seat design, uses only simple electronics, andmonitors gradations of pressure.

Such a device could also help reduce falls due to unassisted attempts torise out of a wheelchair on the part of patients with dementia. Anunassisted attempt would activate an alarm. It is currently veryimportant in nursing home practice to use minimal restraint (e.g., wristor waist straps). Thus, this device could reduce falls, a common causeof morbidity in nursing homes, and at the same time minimize physicalrestraint.

In terms of rehabilitation, many patients immediately after stroke areimpulsive and unsafe. Pressure detection could electronically cue themto lock wheelchair brakes prior to rising, and thus promote safetyawareness and reduce needless injury. All these issues could besuccessfully addressed by a wheelchair embodiment of my invention.

G. Pressure Pad Embodiment

These consist of sets of circular, elliptical or rectangular laminatesof Carbon impregnated silicone rubber and PPT. They would be used forisometric strength training (i.e., other than grip strength) in thephysical or occupational therapy gym. It may inspire creativity withinthese disciplines and be the source of further applications.

OBJECTS OF THE PRESENT INVENTION

Thus, several objects and advantages of the present invention include:

1) Flexibility, such that it may be incorporated into weight measuringshoes, cast boots, wheelchairs, devices to measure grip or pinch, ordirectly incorporated within orthotics.

2) Low projected cost due to simplicity of construction.

3) Unobtrusiveness (it can be made less than 0.16 cm. or 1/8 of an inchthick).

4) Durability, as the only moving part is the compressible plastic foam,which returns to its original thickness after loading after even monthsof continuous use.

5) Simplicity of processing. Weight is integrated over any range desiredto obtain a total weight.

6) Inherently linear output.

7) Simplicity of electronic processing due to 5 and 6.

8) Accuracy to the extent necessary in a clinical setting.

9) Ability to obtain weight applied through large, irregular surfaces aseasily as through small, regular ones: That is, ability to integrateweight over area.

Because of these advantages many embodiments suggest themselves:

1) Foot weight bearing feedback

a) after orthopedic procedures or fractures.

b) after stroke to facilitate re-learning of weight shifting, thenambulation.

c) facilitate paraplegic ambulation.

d) facilitate ambulation of amputees with prosthetic limbs.

2) Upper extremity feedback, such as to provide biofeedback or grip orpinch strength.

3) Application directly within orthotics.

a) correct tendency towards gait deviations in cerebral palsy patients.

4) Application within assist devices, such as crutches.

5) Wheelchair seat pressure feedback.

a) electronic cuing to weight shift in spinal injured patients prone topressure ulcers.

b) alarm to nursing staff that patients on fall precautions haveimpulsively risen from the wheelchair.

c) cue patient to lock wheelchair breaks before rising.

6) Flexible "pressure pads" with general use within the physical oroccupational gym, for muscle strengthening or endurance programs.

Other objects and features of the present invention will become apparentfrom the following detailed description considered in connection withthe accompanying drawings. It is understood, however, that the drawingsare designed for purposes of illustration only, and not as a definitionof the limits of the invention for which reference should be made to theappending claims.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a enlarged perspective view of the cast boot embodiment withina cast boot and associated electronics module;

FIG. 2 is a top and side plan view of the cast boot embodiment;

FIG. 3 is an exploded perspective view of the cast boot embodiment;

FIG. 4 is a graph of weight versus capacitance for the cast bootembodiment;

FIG. 5 is a perspective view of the shoe embodiment being used in atypical application;

FIG. 6 is a top and side plan view of the shoe embodiment;

FIG. 7 is an exploded perspective view of the shoe embodiment;

FIG. 8 is an exploded perspective view of the ankle brace embodiment;

FIG. 9 is an exploded perspective view of the ankle brace embodiment;

FIG. 10 is a plan view of the grip strength embodiment being used in ahuman hand;

FIG. 11 is a perspective view of the grip strength embodiment outsidethe hand;

FIG. 12 is an exploded perspective view of the grip strength embodiment;

FIG. 13 is a front elevation of an axillary crutch and axillary crutchembodiment;

FIG. 14 is an exploded perspective view of the axillary crutchembodiment within the top of the crutch;

FIG. 15 is an exploded perspective view of the axillary crutchembodiment itself;

FIG. 16 is a perspective view of a wheelchair with the wheelchairembodiment within the seat;

FIG. 17 is a top and side plan view of the wheelchair embodiment;

FIG. 18 is an exploded perspective view of the wheelchair embodiment;

FIG. 19 is a perspective view of the pressure pad embodiment; and

FIG. 20 is an exploded perspective view of the pressure pad embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A. Cast Boot Bodiment

The cast boot embodiment is illustrated in FIGS. 1-4. FIGS. 1 affords anoverview of the device. The cast boot (52) is preferably an oversizedcanvas shoe with an open upper surface, and fastens over the foot orcast with a Velcro® strap (53). Also, it provides support and protectionto the patient's foot.

The weight detecting element (50) sits on the upper surface of the castboot. It is preferably very similar in shape to this platform, and notpermanently glued or fastened to it. As such, it is easily installed andremoved by the physical therapist. The device connects toga wire (62),then to an electronic module (64) placed on the cast boot, belt, pocketor other convenient place on the patient by means of a clip (82).Preferably, attached to the electronics module 64 is an adjustment knob(80), and an on/off switch (90). Additionally, there is preferably apiezoelectric buzzer within module 64 (not shown).

The general size and shape of the device is best shown with reference toFIG. 2 which shows top and side views. Device 50 is preferably flat,planar, flexible and rectangular with rounded ends. For a `medium` castboot, it is preferably a maximum of 9.5 cm. wide at the forefoot (54),tapering to 8.5 cm wide at the hindfoot (56). Additionally, it ispreferably 24.5 cm. long and 0.3 cm (1/8 inch) thick.

The method of fabrication is best described with reference to FIG. 3. Itpreferably consists of five layers. The top layer (66) is thin rubber.The next layer down (68) is steel mesh. The perimeter of this layer (69)is preferably recessed 0.7 cm from the perimeter of overall device (73).Mesh area and gauge are preferably similar to conventional windowscreen. Posteriorly, a wire (69) connects with the mesh, and ispreferably soldered 2 cm. from one corner.

Below the mesh is a layer of polyurethane foam, as previouslycharacterized and preferably PPT® polyurethane foam (72), preferably0.16 cm. (1/16 inch) thick. Below the PPT® is another layer of steelmesh (74), substantially identical to item 68. However, the wire (75)preferably connects 2 cm from the other rear corner of the mesh. Thus,the solder points of the wires are not directly beneath each other.Below this is the final layer (78), rubber sheet essentially identicalto item 66.

All parts are preferably glued together with Contact Spray Adhesive(U.S. Pat. No. 4,401,272, 3M Company, St. Paul, Minn.). Thisneoprene-based product bonds well to rubber, plastic foam and metal. Thefive layers thus form a strong, but flexible interconnecting bond. Thisfacilitates prolonged clinical use without loss of properties.

The capacitive transducer is created when the dense polyurethane foam(72) is placed between metal plates (68 and 74). As weight is appliedthe foam is compressed, decreasing inter-plate distance and increasingcapacitance. Conversely, as the weight is removed the inter-platedistance and capacitance return to their original values. To demonstrateboth stable baseline and linearity, a cast boot embodiment of the abovespecifications was tested for its pressure-capacitance profile. FIG. 4is a graph describing these results. Zero to 160 lbs of weight wereapplied over 5 areas of weight application: 9, 16, 30.2 and 40.2 cm².(For reference, the area of the average adult great toe is about 6 cm²,heel 30 cm², and forefoot 45 cm²). Clearly, a high degree of linearityis demonstrated over a wide range of compression of the foam.

In a conventional manner, electronics module (64) converts capacitancelinearly to voltage. Of note, top plate 68 is connected to ground tolargely neutralize stray capacitance of the subject from affecting thissignal. This voltage is internally compared with a trigger voltagepreferably set by knob 80. This knob is set in conjunction with placingthe patient's foot on a scale prior to ambulation. When weight exceedsthis preset trigger a piezoelectric buzzer within the electronics moduleprovides feedback to the patient and physical therapist. In this mannerthe patient is informed about toe touch, and partial or full weightbearing on the affected leg.

B. Shoe Embodiment

The shoe embodiment in a typical application is illustrated in FIGS.5-7. In FIG. 5 a paraplegic is practicing reciprocal gait by weightshifting between extremities when both legs are anaesthetic. Transducer(100) is preferably beneath the inlay of an orthopedic oxford shoe onthe right foot. Wires (114, 117) extend to the waist on the right sidewhere electronic module (128) is hooked onto the belt. There is asimilar configuration on the left side (not shown). The electronicsmodule is similar in static design to the cast boot module, and has atrigger mode, and on/off switch. Inside, there is a small vibratingmotor that imparts vibratory feedback to the patient. A wireless designcould also be employed (not shown). This would further improve cosmoses.

The shape and thickness of the shoe embodiment is best illustrated withreference to FIG. 6. The device is preferably thin, planar, flexible andsubstantially flat.

The fabrication of this device is best described with reference to FIG.7. It preferably consists of five layers. These are from top to bottom,gum rubber upper piece (104), top layer of steel mesh (106), layer ofPPT (108), bottom layer of steel mesh (110), and gum rubber bottom piece(111). Steel mesh layers 106 and 110 are preferably recessed 0.7 cminside the perimeter of the overall device. Steel mesh top plate (106)connects to a wire (114) near the front of arch (112). Similarly, steelmesh bottom plate (110) connects to a wire (117) near the rear of arch112. These two wires connect to electronics module (128).

The calibration and operation of the shoe embodiment is similar to thecast boot embodiment. However, the feedback is preferably tactileinstead of auditory. The rationale is that the audible alarm may bedistracting and embarrassing for a highly functioning patient in a workor social setting. Calibration of trigger level is done by the patientby setting a threshold in conjunction with putting the foot on a scale,and varying trigger adjustment knob. Vibration intensity is all or noneabove this threshold. Alternatively, it can be wired to continuouslyincrease in intensity with weight increase above a threshold. Theelectronic design of this device is conventional. In preliminary studiesan active adult male wore this device for several hours. A stablebaseline capacitance, and stable capacitance increase per unit weightincrease, were noted throughout the experiment.

C. Ankle Brace Embodiment

FIGS. 8 and 9 refer to this device. FIG. 8 shows an overall perspectiveview. The ankle brace embodiment (152) preferably fits between the anklefoot orthosis (AFO) (150) and shoe (154). (Very often the patient withcerebral palsy has excess tone in the calf muscles. The AFO helps tocontrol ankle movement in the face of this muscle imbalance). Wires(154, 156) go to an electronics module. This module is identical inappearance to the module 64 connected to the cast boot embodiment.Therefore, it is not specifically illustrated here. (As an alternative,transducer 152 and module 64 could be fabricated as integral parts ofAFO 150).

FIG. 9 best illustrates construction of this device. As above, there arepreferably five layers. They are (from top to bottom) thin rubber toppiece (160), steel mesh layer (162), PPT® polyurethane foam layer (164),second steel mesh layer (166) and rubber bottom piece (168). Wires 154and 156 go to the electronics module. Each layer is preferably ofidentical thickness and composition as the cast boot embodiment.

The goal of this device is to defeat the tendency of children withcerebral palsy to "toe walk" by constant reminder to plant the heelfirst after the swing phase of gait. Unfortunately, toe walkingfrequently persists even if AFOs are worn. Prior to an exercise sessionthe device is calibrated to elicit a sound at or near the patient's fullweight. During the exercise session normal heel strike causes atransient increase in capacitance above threshold causing an audiblereward signal to be emitted. However, if the foot comes down in equinus(i.e., toe strike instead of heel strike) the capacitance increase doesnot exceed a preset trigger and no beep is elicited. This is becauseafter toe strike weight is never concentrated in the heel. Thisinvention will thus assist a cerebral palsy patient learn normal heelstrike during gait.

D. Grip Strength Detector Embodiment

FIGS. 10-12 refer to the grip strength embodiment. FIG. 10 illustratesthe isometric grip strength detector embodiment (200) in use. It ispreferably cylindrical of such dimensions as to fit snugly inside theclenched adult human hand (202). An electronics module (252) ispreferably incorporated inside the cylindrical transducer. Thresholdstrength adjustment knob (254) has a built-in on/off switch. Thresholdstrength can be set preferably from 0 to 60 lbs on this embodiment usingpreset numerical settings (256). FIG. 11 shows the general configurationof the device. It is preferably 12 cm (4.25 inches) long and 3.7 cm (1.5inches) in diameter.

Construction of the grip strength detector embodiment 200 is bestdescribed with reference to FIG. 12 where the five layers are shown. Forclarity, the four outer layers are divided into upper and lower halvesand projected above and below the central cylinder. However, thecylinders are preferably continuous and concentric. Housed within device200 is the electronics module 252.

The innermost layer (204) of embodiment 200 is preferably cellulosebutyrate tubing of inner diameter of 2.54 cm (1 inch) and outer diameterof 3.17 cm (11/4 inch). Going outward, the next layer (206) is acylinder of steel mesh similar to window screen. It is preferablysoldered at point 208 to a wire (210) and reinforced with epoxy cement.The mesh cylinder (206) preferably fits snugly over the cellulose tubing(204), and is 144 cm² in area. The next layer (220) going outward isPPT® polyurethane foam preferably 0.16 cm (1/16 inch) thick. Above thisis a second layer of steel mesh (226) slightly larger in diameter thanitem 206. However, there are preferably four grooves (228,230,232,234)about 1 mm wide that extend from edge 260 to 4 mm from the other edge ofthe cylinder. Thus at end 260 the mesh is preferably divided into fourseparate sections. However, at end 262 these sections are electricallycontinuous. Solder connection 246 connects this layer of mesh to wire(248). Solder connection 246 is also reinforced with epoxy cement. Theoutermost layer (250) is preferably made up of gum rubber. Wires 210 and248 connect to electronics module 252 as indicated by the dashes. Alllayers are preferably glued together with Contact Spray Adhesive (3Mcompany). (However, other contact adhesives may be used).

To use, the patient grasps transducer 200 and applies forcecentripetally (i.e., toward the center) with the fingers and thumb.Grooves 228 (etc.) allow the outer mesh element 226 to contract indiameter as distance between plates 226 and 206 decreases proportionalto the force applied. Secondarily, capacitance increases. In testing,capacitance increases linearly and reproducibly from a stable baseline.This baseline is 78 pF. (no force applied). Capacitance increases to 148pF. with 70 lbs. applied.

Grip strength is calibrated in pounds prior clinical use. The poundssettings are preferably printed around the adjustment knob. The desiredthreshold force is set by the occupational therapist and patient usesthe device as instructed. A sound emitted above a threshold serves as areward for having achieved a given compression. (Alternatively, it couldbe an alarm to prevent exceeding a given compression). Experience willdetermine if subtle baseline shifts occur with heavy use. If so,periodic re-calibration may be required.

Although a cylindrical grasp is illustrated, there are several otherimportant grasps. These include pinch (i.e, thumb against the lateralindex finger) or ball grasp (i.e., hand grasping a ball). Thisinvention, as a flexible laminate, may accommodate any of these shapesand provide feedback for nearly any type of grasp.

E. Axillary Crutch Device

This embodiment is shown in FIGS. 13-15. An overview of the size, shapeand placement of this embodiment is illustrated with reference to FIGS.13 and 14. An axillary crutch (250) preferably has a rubber cap (252),which fits over a convex wood top-piece (254). From this, long,vertical, bowed wood members (256,258) converge on the rubber base(260). A transducer (262) is preferably interposed between rubber cap252 and the top piece 254. The overall perimeter of transducer 262 ispreferably the same as this top-piece, and it fits snugly inside therubber cap. A set of two wires from the transducer (264, 266) go to anelectronic module (268) fastened near the top of one of these twovertical wood members. In the preferred embodiment, module 268 has noexternal controls. Internally, there is an on-off switch, weight triggercontrol adjustment and piezoelectric alarm device similar to the castboot electronics module (64).

The construction of this transducer is best described with reference toFIG. 15. There are preferably six layers from top to bottom. Anuppermost layer (280) is preferably made of Orthoplast®, a 1/8 inchthick semi-rigid plastic the shape of which may be molded at thetemperature of boiling water. Below this is preferably a copper foillayer (282), of the same length and width as the Orthoplast®. Wire 264is soldered from foil 282 to the electronics module 268. Below this is alayer of polyurethane foam elements (284). Preferably there are 24 suchrectangular pieces, about 0.95 by 0.50 cm. (0.375 by 0.2 inches),regularly spaced in a staggered arrangement. These preferably coverabout 33% of the area between the adjacent layers. Below this ispreferably an oversized layer of thin rubber insulation (286). Belowthis is copper foil (288) is essentially identical to foil item 282.Wire 266 connects this layer to electronics module 268. The bottom layeris preferably Orthoplast® (290), of very similar dimensions to thecorresponding top layer 280. All layers are preferably glued togetherwith Neoprene contact adhesive. The overhanging edges of rubber layer286 preferably fold under the Orthoplast® layer 290. This is so copperlayers 282 and 288 will not short together.

In operation, a reference trigger weight (voltage) is set internallywithin the electronics module. An audible alarm sounds if the weightapplied through the axilla exceeds a this reference weight. The accuracyis periodically checked by a physical therapist using an inexpensivescale, and re-adjusted if necessary.

By way of theory, the smaller area of polyurethane foam improves thesensitivity of the device. Sensitivity may be defined as the ratio ofcapacitance change to baseline capacitance. With one third of the areacovered, sensitivity increases three times. Semi-rigid plastic members280 and 290 are then used to more evenly transmit this weight over theset of compressible elements 284. By this method, capacitance increaseslinearly from a baseline of 70 pF. with 0 weight applied, to 140 pF.with 70 lbs. applied. A similar result would be obtained if a lowerdensity of polyurethane foam were available. Then, the foam layer couldcover the entire area between the plates, and eliminate the need forseparate reinforcement and insulation layers.

F. Wheelchair Pressure Detector Embodiment

FIGS. 16-18 refer to the wheelchair embodiment of the present invention.FIG. 16 affords an overview of this device. A pressure detector (300) ispreferably placed in the sling seat of a standard wheelchair (301). Thisseat consists of and upper and lower vinyl member joined on the twosides. Transducer 300 fits easily between the vinyl panels that make upthis seat. As it is preferably thin, flexible and smooth, it conforms tothe curvature of the seat with the wheelchair folded, open or occupied,and it adds no irregularities to the seat. In other embodiments (notillustrated) the device may be placed between a sling or solid seat andspecial cushions, including a Jay® and Roho® cushion. Lead 302 connectstransducer 300 to electronics module 304 located on the backrest.(Module 304 is physically equivalent to cast boot electronics module64). In other embodiments leads may extend to toggle brake 306.

The size and shape of transducer 300 is more exactly illustrated in FIG.17, which shows top and side views. It is preferably square, 33 cm. (14inches) on a side and is 0.3 cm. (1/8 inch) thick. Moving inward, thedashed concentric outlines and perimeters are best presented in theexploded view, shown in FIG. 18.

The construction of transducer 300 is best illustrated in FIG. 18. Aswith previous embodiments, there are preferably five layers. The toplayer (320) is preferably rubber sheet. Just below this is the upperlayer of steel mesh (322). It is similar to window screen in appearanceand is preferably 33.6 cm. (12.5 inches) on a side. It is connected towire (324), thence to electronics module 304 (not shown on this figure).Preferably, below this is a layer of PPT® foam (328), 0.16 cm (1/16inch) thick, and 31 cm (13 inches) on a side. Below this is a plethoraof steel mesh elements (332), preferably forming a "daisy chain" of meshelements. The chain preferably doubles back after every 3 elements toform 6 rows of elements. Preferably, square 336 and 16 others areconnected by two wires, and the end square 340 and one other, areconnected by one wire, to adjacent squares. All told, 18 substantiallyidentical squares are connected electrically in series. Representativesquares 336 and 340 are preferably 2.54 cm. (1 inch) on a side. Theircenters are preferably 10.2 cm (4 inches) distant from their immediateneighbors along a row or column, and 7.0 cm along a diagonal. All othersquares are similarly related to their neighbors. A wire (348) connectsthe set 332 to the module 304. The bottom layer is preferably anotherpiece of thin gum rubber (352). As with other embodiments, the layersare preferably glued together with Neoprene Contact adhesive.

Device 300 is a capacitive transducer that preferably works in similarmanner as cast boot device 50. However, weight is applied over a muchlarger in area than the cast boot. Because capacitance is directlyproportional to plate area, a test device with both plates full size(i.e., the size of mesh element 322) had a baseline capacitance of 800pf. This is compared to 215 pF. for the cast boot device (50) (see FIG.4). Sensitivity is here defined as the ratio of capacitance change tobaseline capacitance. Because of the larger baseline capacitance, the20% sensitivity of the cast boot device decreased to only 5% for theinitial wheelchair prototype. By sampling weight over 18 equidistantmuch smaller areas sensitivity improved to 35% for the preferredembodiment. The only caveat for sampling of discrete points is thatweight be relatively evenly applied over the entire surface. For thegluteal region this assumption is reasonable.

In terms of function, electronics module 64 is modified internally.Here, an increase in weight (voltage) above the trigger set by knob 80turns off an alarm. Conversely, a decrease in voltage (weight) below thetrigger turns on the alarm. This is opposite the alarm sequence of thecast boot device.

To operate, the nurse or physical therapist has the goal of beingnotified if a patient is rising impulsively out of the wheelchair. Thetrigger is thus preferably set to about one quarter the patient's weightwith knob 80. The device is engaged by turning on switch 90. Thus, ifthe patient rises, the alarm will sound and help summoned. (In someembodiments, a delay may be built in so the patient may weight-shiftwithout sounding the alarm).

The other two uses mentioned in the beginning of this section may beaccomplished by modification of electronics only. For paraplegics, aweight-shift off the gluteal region would reset an timer circuit. If asecond weight-shift did not occur with a preset interval, an alarm wouldsound. For the stroke patient, setting both toggle brakes woulddisengage the alarm circuit that would otherwise sound on standing ortransferring.

Lastly, the upper plate 322 is grounded to prevent stray capacitancefrom affecting weight measurement.

G. Pressure Pad Embodiment

FIGS. 19 and 20 refer to this device. FIG. 19 is an overview. Thepressure pad embodiment (400) is more flexible than those discussedabove by virtue of carbon impregnated silicone rubber (CISR) as theconductive layer instead of wire mesh. Preferably, the pressure pad iscircular. It is preferably connected by three wires (items 402, 404, and406) to an electronics module. This module is otherwise identical toelectronics module 62 (cast boot embodiment). Therefore, it is notshown.

FIG. 20 best illustrates construction of the pressure pad embodiment400. A cloth upper surface 410 preferably fits above a layer of carbonimpregnated silicone rubber (412). The latter is preferably 0.16 cm.(1/16 inch) thick. (However, 0.08 cm. may also be employed to increaseflexibility). Below this preferably is a layer of PPT® polyurethane foam(414). Below this is preferably a central layer of carbon impregnatedsilicone rubber (416), and below this is another layer of PPT® (418).Below this is the lowest layer of carbon impregnated silicone rubber(420). Below layer 420 is the bottom cloth layer 424. Layers arepreferably glued with Neoprene cement, after roughening apposingsurfaces with sandpaper, or other appropriate abrasive. Wires 402, 404and 406 are preferably glued to conductive layers 412, 416 and 420respectively, by means of a conductive silicone rubber cement. PPT®layers 414 and 418 and cloth layers 410 and 424 are preferably ofequivalent diameter and 0.5 cm greater in diameter than silicone rubberlayers 412, 416 and 420.

Application of weight increases capacitance by compressing foam layers414 and 418. This brings closer conductive layers 412, 416 and 420.Testing reveals a linear relationship between weight and capacitance.This is true for various areas of weight application, similar to thatshown in FIG. 4.

In previously described embodiments there was only one capacitive layer,made up of two conductive layers with interposed dielectric foam. Inthose, only one surface, the layer closest to the patient, waselectrically shielded. Only this side could face the patient withoutproducing spurious readings. Those embodiments are said to be"polarized". For this embodiment there are essentially two capacitivelayers in parallel. Both outer surfaces are grounded (i.e., shielded).Thus, either surface may face the patient without causing spuriousreadings. This embodiment is thus "non-polarized".

CISR is a suspension of small carbon particles within silicone rubber.Its conductivity is directly related to frequency; it is conductive toAC but not DC signals. Since electronics module 408 measures capacitanceusing AC signals, rubber layers behave as would metal conductors in thiscontext. This sensor is therefore preferably made without metal,excluding the wires (402 404 or 406).

With respect to application, this embodiment is for general isometricstrengthening (i.e., other than grip strengthening). The therapist mayposition the device in front of the ankle and behind an immovablesurface for quadriceps strengthening, behind the ankle and in front ofan immovable surface for hamstring strengthening, or between the kneeand wall for gluteus medius strengthening. There may be occasions wherethe both surfaces are in contact with the patient, as if placed betweenthe knees for adductor strengthening. In this case double shieldingprevents spurious readings. The patient gains reward for reachingcertain strength goals by auditory feedback.

In other embodiments, the device could be made a rectangular shape ofvarious dimensions. Non-polarized pads could be used for strength andendurance training. Non-polarized rectangular pads could be made to fitvarious cast boot sizes, and take the place of the cast boot embodiment.Non-polarized inner-sole shaped devices could take the place of the shoeembodiment. Non-polarized circular devices could take the place of theAFO embodiment. Further, very thin (i.e., 0.8 cm. or less) conductivesilicone rubber layers would make for very flexible, resilient devicesbetween irregular surfaces.

Additional improvements could be made to the electronic module.Continuous digital display of weight in pounds is possible. This wouldbe very helpful for weight training. Beyond this, a portable digitalscale for the mass market could be made in the following way: Tworectangular or inner-sole shaped pressure pads could be positioned on aflexible plastic screen. These pads would be connected to a smallelectronics module with digital readout in pounds, also positioned onthe screen For a weight conscious person "always on the move", thedevice could be unrolled onto a hard surface such as a hotel bathroomfloor and re-rolled for travel.

Accordingly, the reader will see that this invention is a capacitivetransducer with wide potential application in the area of rehabilitationmedicine and also the mass market for exercise or toning devices. Inoperation the device measures the capacitance change between two metalscreens preferably separated by a open cell polyurethane foam dielectricknown as PPT®. In some embodiments, the metal screen is replaced byelastic conductive silicone rubber. As weight is applied, the foamcompresses and capacitance increases.

Although the concept of the capacitive transduction is not new, thisdevice possesses many important improved properties. These includingunsurpassed resiliency and durability of the dielectric. Response islinear and reproducible. It additionally has the ability to measureweight accurately when applied over a large or small area via irregularsurfaces. Construction is simple, and readily available electronics maybe used with very low current drain. Small batteries may thus powerthese devices over a long period of time.

The materials that create these improvements have been well known in thefield for some time. However, no one has thought to apply them to solvethe important problem of how to measure limb loading. Therefore theseimprovements are not obvious.

Additionally, the prior art has so far been totally unsuccessful inmeeting the need for load monitoring. Very little of the prior art isactually in use. This is true even though the potential market is huge.

This invention is broad in application. It has many specific embodimentsin the areas of shoes, prosthetics, orthotics, assist devices andwheelchairs. The invention would be of interest to the physiatrist,orthopedic or plastic surgeon, physical and occupational therapist. Itswide range of potential use stems from thin size, and ability to assumedifferent shapes.

The device is simple for both patient and practitioner to operate. Thus,this invention may overcome the natural resistance of thesenon-engineers to "anything electronic". This is unfortunately anotherimportant reason why the prior art involving computers has been rejectedby the rehabilitation and medical community.

Having gained wide acceptance, devices based on the present inventionwould be medically important. They would: 1) help insure accuratefracture healing by more accurate implementation of the weight bearingprescription; 2) improve safety of re-learned ambulation ofparaparetics, hemiparetics, and amputees by improving sensory awareness;3) improve ambulation of individuals with predominantly sensory loss; 4)improve upper extremity strength and reduce pain in arthritics andstroke patients; 5) reduce pressure sores in paraplegics due toprolonged wheelchair sitting; 6) reduce the rate of falls in strokepatients due to impulsively rising out of the wheelchair; and 7) improveeffectiveness of strength or endurance training.

Although the description above contains many specificities, these shouldnot be construed as limiting the scope of the invention but as merelyproviding illustrations of some of the presently preferred embodimentsof this invention. For instance, any of the embodiments may be madewireless. The electronic modules may be made very small using surfacemount or other technologies of manufacture. Thus, complete embodimentsmay be incorporated within orthotics, prosthetics or assist devices.There may be other means used to provide biofeedback to the patientbesides sound or vibration.

Thus, the scope of the invention should be determined by the appendedclaims and their legal equivalents, rather than by the examples given.

What is claimed:
 1. A biofeedback sensor comprising:(a) dielectric meanshaving a first and second side, the dielectric having a hysteresis andcreep that approach zero and having a high resilience, sensitivity anddynamic response, wherein the percent loss of dielectric means thicknesschange is negligible when subjected to cyclic compression and shearcompression tests; (b) a first conductor and a second conductor, whereinsaid first conductor is secured to said first side of said dielectricmeans and said second conductor is secured to said second side of saiddielectric means; (c) electronic means connected to said first conductorand said second conductor, wherein upon compression of said dielectricmeans, said electronic means detects and measures linear response in thecapacitance through the change in separation between said first andsecond sides of said dielectric means and compares said capacitance witha preset value set within said electronic means; and (d) feedback meansfor imparting information to a user according to the relation betweensaid capacitance and said preset value, said second conductor has aplurality of mesh elements connected with one another.